• ASI Mount
  • Getting the best performance from my AM5

@w7ay , Chen, your comment on the weight distribution caught my eye.

I hope I can explain this well. Instead of piggybacking the guide scope and guide camera on top of the imaging OTA, would it be better---with a longer OTA and dovetail--- to mount the guide scope and camera UNDER the imaging OTA, and thereby closer to the mount axis? So instead of piggybacking, over-under shotgunning with the guide scope the lower barrel?

Thanks for all your wonderful explanations and expertise. I spent several hours reading through this thread.

  • w7ay replied to this.

    SteveV So instead of piggybacking, over-under shotgunning with the guide scope the lower barrel?

    That sounds like a great idea (as long as it is close to the declination axis instead of way out near the dew shield). It is kinda like rotating a Newtonian tube so that the camera (weight) is towards ground, instead of pointed at the sky.

    Chen

      w7ay hi Chen, I am trying to figure out why my mount does not seem to be responsive to any duration change or exposure length change. I still get the randon 2” spike in RA but my guiding is always near 0.7rms no matter what I do to try to improve it. If I run at 1s at 500ms on dec and ra at 40% aggression I can change it to 80ms and absolutely no difference. I can run at 0.5s up to 3s and always the same 0.7 to 0.8 rms. Asiair always just picks 6-8 stars, unsaturated but not very sharp stars in the box. I am starting to think that maybe it is my guide scope sharpness that may be contributing to the inability for the program to guide better. My slope was 0.16 so fairly good for zwo.
      Even bumping things up to 0.5s gets me the same sawtooth pattern 0.7 average rms with 100ms pulses. Any thoughts? Thanks. I would like to smooth out my graph and try for 0.5 average. All my calibrations are within 30 degrees of the meridian and i using my 290mm mini on a 50mm f3.2 guide scope. It has got me baffled why it still seems jagged even with 100mm pulses and less. Better guide scope needed or do my calibrations on target rather than near the meridian? Right now it seems that nothing really improves my guiding. I do not expect miracles but it would be nice to figure out the weakest link! TIA.

      • w7ay replied to this.

        Kevin_A If I run at 1s at 500ms on dec and ra at 40% aggression I can change it to 80ms and absolutely no difference.

        Hi Kevin, remember that the 80ms number is the max duration of a pulse. If the autoguider never asked for more than 40ms (for example), then setting it to 80ms or 2000ms will not change anything, since there is nothing to limit in that case.

        I can run at 0.5s up to 3s and always the same 0.7 to 0.8 rms.

        Yeah, this is the weird part.

        With the rapid changes in the periodic error (i.e., large slopes caused by the higher harmonics), the error should improve from the 0.7" down to the 0.3" to 0.4" range that your mount should be capable of.

        A 3 second guide exposure would allow your 0.16" slope to move by 0.16 x 3 arc seconds before any correction is issued, or around 0.5 arc seconds; and that is just for the RA axis.

        If the declination axis has the same amount (but independent) of 0.5" RMS error, then the total would come to be 0.707" total RMS... (RMS is the square root of the sum of squares), or close to what you are seeing.

        Lopping that exposure time down to 1 second should have limited the excursion from 0.5" down to 0.16", and make the error be limited by other things than the mount's PE itself.

        The fact that you are not seeing an improvement points to something else being the "tallest tent pole." I.e., something else is causing a 0.5" type error (in the RA axis) so that shortening the exposure time did not improve anything.

        Asiair always just picks 6-8 stars, unsaturated but not very sharp stars in the box.

        And that may be the problem.

        Remember that some time back, I had shown a number of bar graphs showing the "typical" SNR distribution of stars. The ASIAIR uses SNR as a weight to determine the average centroid. And, given 6 stars, it is probably just the equivalent of using the uniformly weighted centroid of only 2 stars. I.e., you are getting very little improvement from multiple stars, and that could be the "tallest tent pole."

        On an ASIAIR with a 55mm aperture guide scope at 200mm focal length (f/3.6) and an ASI178MM at a gain of 250 (analog gain of 25 dB) and an exposure time of 0.5 seconds, I can consistently get 12 stars from it, even in the star starved region around M81/M82 where I was playing around last week when clouds were thinner.

        Notice a couple of things in this choice. The guide scope is actually a Borg 55FL with its reducer, chosen to give me a large aperture (and flat enough to cover the camera). The ASI178MM was chosen to give me a large FOV (if all else is equal, the larger FOV means more stars to pick from). The camera gain is pushed enough so that singularly bright stars will saturate, and causes the one or two bright stars to not get included in the centroid average (otherwise the SNR weighting will reduce to just using that single one or two bright star for guiding).

        So, I would recommend tuning your guide system so that you can get all 12 stars that ASIAIR will process (the most the ASIAIR will use; guide systems like DONUTS will use all stars in the FOV that it can see, and DONUTS also uses uniform weighting instead of SNR weighting).

        If the centroid measurement is coarse, then you are limited by centroid measurements, not by the feedback loop.

        Check too that your stars are not distorted towards the edge of the field. I don't think that is your primary problem, but it can be as you tune the guide system.

        You may have been too timid with pushing up the gain to get the stars. Don't be. With the short exposure times (i.e., shorter than 10 seconds), you can pump the gain up quite a bit before the dark noise dominates over the read noise.

        After you have the guide scope tuned to consistently get 12 stars (while ignoring saturated stars) the next thing to do is to reduce the guide rate to 0.25x sidereal on ASIAIR (the ASIAIR will not let you go below that). It was a serendipitous discovery when looking at the sawtooth graph, but so far has served me well since I started using it.

        By the way Kevin, on a different topic, I took my own advice about drilling the EAF to keep it from falling below the declination plate for both my FSQ-85 (first image) and my FOA-60Q EAFs (second image).

        So, I have at least 4 EAFs now that are mounted this "direct mounting" way.

        Chen

          w7ay thanks, i will look over the guide rate as I was under the impression that asiair locks out AM5 mounts from adjuting it from 0.5. Unity gain is 110 and I had it set to 200 as I thought I might not have enough stars. I will check again tonight. I was using a 60mm svbony generic scope last time and back using my Orion 50mm as with testing it turns out to be muh sharper than the cruddy svbony 60mm. I might try an askar fma180 pro soon as maybe that will improve my stars and not break the bank too much.

            Kevin_A Hi Kevin, maybe it's listed in an earlier post, but what guide camera are you using? My ASI178 consistently grabs 10 to 12 stars (using the AA+), as long as there are that many to choose from in my FOV. MY ASI120 rarely captured more than 6.

            Edit: I get the same random spikes that you are describing. Regardless of duration, aggression, etc. They generally come and go within 2 frames of guiding, and since my rig has been working well, I see no detectable affect in my images.

              w7ay After you have the guide scope tuned to consistently get 12 stars (while ignoring saturated stars) the next thing to do is to reduce the guide rate to 0.25x sidereal on ASIAIR (the ASIAIR will not let you go below that). It was a serendipitous discovery when looking at the sawtooth graph, but so far has served me well since I started using it.

              Unfortunately when using a AM5, there are no options in asiair to adjust rates. When I used my Gem45 mount there was the usual options… but, no more from looking everywhere in asiair.

              Kevin_A Best I know, 0.5 is the only available guide rate using the ASIAir. However, maybe the newly released firmware has changed that. Not sure. See item 5 below.

              "From V1.1.2 to V1.3.0, we made these changes:
              1.adding meridian settings
              2.adding altitude limits
              3.adding a fixed park position
              4.optimizing cable winding problem in the Alt/Az mode
              5.memorizing custom guide rate
              6.fixing DST/LST misunderstanding
              7.fixing problem that track light off after GOTO
              8.adding multi star alignment (differ from polynomical optimization )
              9.fixing other bugs"

              Edit: If using a PC, guide rates are adjustable.

                KC_Astro_Mutt Best I know, 0.5 is the only available guide rate using the ASIAir.

                FWIW, this is what my mount shows:

                The command protocol of the RST-135 has a resolution of 0.01x sidereal rates, but the ASIAIR limits the guide rates to four values: 0.25, 0.5, 0.75 and 0.9x. Problem as usual with ZWO crippling the better equipment the ASIAIR connects to. I don't know why it does not allow the user to select what they want, instead of the limited knowledge of ZWO's coders on what good guiding requires.

                Chen

                  w7ay with a 55mm aperture guide scope at 200mm focal length (f/3.6) and an ASI178MM at a gain of 250 (analog gain of 25 dB) and an exposure time of 0.5 seconds,

                  Chen,
                  How would that translate to phd2 where gain is a percentage? I have the 178MM and have used it with both 50 and 60mm guide scopes and can never get more than 9 stars from 0.5 all the way to 2 sec intervals of exposure. I tried gains from 30% all the way to 90%. I use the default values for SNR.
                  I read in the phd2 manual it can go up to 12 stars but never managed to get it to do so even if I see lots more stars of similar brightness in the field of view to the ones phd2 selected.

                  Thank you

                  w7ay yes, mine is exactly the same minus the guiding speed!

                  • w7ay replied to this.

                    Kevin_A yes, mine is exactly the same minus the guiding speed!

                    ZWO is crippling even their own mounts :-).

                    Chen

                      In case you don't believe 12 stars is possible from ASIAIR, here is a screenshot I happened to take down back at the end of April:

                      Notice 0.5 second exposure. Back then, I was using 250mm focal length (the Borg 55FL without the reducer at f/4.5) so notice that the gain is all the way high up at 289 (29 dB!). I am not shy about increasing the gain to get enough stars (until even the dimmer stars saturate and I no longer have any stars left to guide on :-).

                      With the reducer, my FOV went up, thus more stars to pick from. I was a little afraid of the skewed plate scale, but it seems fine to use such a short (200mm) focal length.

                      By the way, I had a little while ago made up a list of guide plate scales assuming different guide scopes and guide cameras:

                      The D2 (diameter squared) give you an idea of how many photons are collected. And the FOV area shows the amount of usable stars, if all elase are equal.

                      If it is hard to read, I can put the text file on my web site.

                      Chen

                        w7ay that made me laugh as it is so true! Only is it adjustable in Phd2… not asiair.
                        Crippled for sure….

                        w7ay In case you don't believe 12 stars is possible from ASIAIR, here is a screenshot I

                        I went back to the Phd2 manual as I was having doubts..Here is an excerpt from the manual:

                        "When this option is enabled, the Auto-select function will identify up to 12 stars in the field of view that have adequate SNR. No more than 9 of these will be used at any one time, but the remainder will be used to replace secondary stars that are lost or rejected for some reason."

                        So only 9 stars are used to guide with Phd2. That's what I am consistently getting. Sooooo the ASIAIR is better in that regard. Take the win guys! ;-)

                        w7ay i bumped up my gain last night on my 290mm mini to 300 which is high gain setting and I was getting 12 stars too. Asiair did however pick 11 good stars and 1 very saturated star… which concerned me a bit. Cheers! Guiding was a bit better at 0.6 to 0.7rms and i am ok with that but without being able to adjust the guide rate it seems crippled as you said!

                        • w7ay replied to this.

                          Kevin_A Asiair did however pick 11 good stars and 1 very saturated star…

                          Use single star mode (click on the star before guiding) to take a look at the ADU (based on 8 bits, on the small window showing a curve through a star) to see if it is saturated -- if it is saturated, ASIAIR should not pick it when autoselecting stars in multi-star mode.

                          If it is really picking a saturated star (useless for determining centroid), you have come across a bug that I have not yet stumbled on in ASIAIR (believe me, there many bugs in the ASIAIR multi-star guiding, including not be able to guide for a couple of hours at a time before you need to stop and restart guiding to retain the reference stars).

                          If you want to exclude a star that is borderline saturating (scintillating between saturation and not saturated), either reduce the gain or increase the gain some more (again, don't be meek with the gain control). The latter is preferred since the SNR distribution of the remaining stars (used as weights of the centroid calculation) may be more even.

                          Don't ever let any saturated star through at any time, even momentarily. The centroid will be completely wrong (jittering all over the place) and because if is so bright, the SNR will make the (wrong) centroid dominate over the centroids of all other stars (i.e., turns into a one-star autoguiding on a saturated star). ASIAIR should already handle this, but I not seen its GUI drop any momentarily saturated star.

                          Chen